However, until now, scientists have been baffled by the spiral tusk protruding from the head of the narwhal, an arctic whale.

Now, they have solved the mystery - and say it is, in effect, a giant sensor.

Scientists believe that the tusk can pick up differences in the whale's environment, the salt content of seawater, and even help it find a mate

THE NARWHAL

The narwhal, or narwhale has the largest canines in the animals kingdon.

The tusk, usually found only the males, can grow to 2.6m.

It projects from the left side of the upper jaw, through the lip and forms a left-handed helix.

The tusk is hollow and weighs only around 10 kg (22 lb).

About one in 500 males has two tusks, which occurs when the right canine, normally small and less straight, grows as well.

The scientists believe that the tusk, usually found only the males, can pick up differences in the whale's environment, like the salt content of seawater, helping the marine mammals to navigate their frigid homes or perhaps find food or a mate.

It can grow to up to 2.6m long.

Martin Nweeia of the Harvard School of Dental Medicine, lead author of the new study published in The Anatomical Record, believe it is rather like a particularly sensitive tooth.

There are channels scattered throughout the tusk's external layer that allow seawater to enter the tooth.

That layer connects to another underneath—called dentin—that also contains small tubes.

Those tubules run to the innermost part of the narwhal's tusk, the pulp, which is full of blood vessels and nerves.

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The nerves run from the base of the narwhal's tusk directly to the brain.

For the study , narwhals were captured and tagged.

'New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment,' the researchers wrote.

'The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue.'

There are channels scattered throughout the tusk's external layer that allow seawater to enter the tooth.

The scientists believe the tusk helps the animals sense changes in the environment which the male puts to advantage when looking for females and detecting if they are ready to mate as well as finding food.

'New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk.